1. Technical Field
The present invention relates to improvements in linear motion bearings for slides and ways. The present invention has particular application to machine tools and robots in which precisely-positionable, low friction movement of heavy loads linearly in a single direction (along a rail or way) is required.
2. Background Art
The prior art has proposed several solutions to linear motion truck and way (rail) assemblies. The solutions generally are unacceptable for many machine tool applications, in view of limited load-carrying capacity or useful bearing life or the high cost of manufacturing such bearings. In some situations low friction movement of heavy loads may be necessary or highly desirable without degrading (wearing) the bearing.
For example, U.S. Pat. No. 4,302,059 issued to Teramachi and entitled "Linear Guide Slide Bearing Unit", discloses one such prior art linear bearing assembly in which recirculating rollers are received in V grooves, with alternate rollers being crossed so that a single set of recirculating rollers provides both horizontal and vertical supporting forces. This design requires rails having unusual configurations and a specially manufactured, high precision bearing element mounting body of an unusual shape for receiving and retaining the rollers. Additionally, the load carrying capacity of such bearing unit as disclosed in this patent is substantially limited inherently by its design.
Another prior art concept is shown in U.S. Pat. No. 4,184,719 issued to Ward and entitled "Machine Rails and Carriages". In this system, a portion of a machine tool is mounted by a carriage having multiple sets of cam-follower type bearings for engaging V shaped track members mounted to opposite sides of an unusually shaped track. Such a track is presumed to be difficult and expensive to manufacture accurately and furthermore requires the addition of separate v-shaped members to be positioned and secured thereto. The cam follower bearings proposed in the carriage mounting the machine tool structure have a limited load carrying capacity and has the further disadvantage that it is relatively expensive to manufacture.
Linear way bearings are manufactured and sold under the name IKO linear way (LWA) bearing. The bearing of this embodiment is a ball bearing which the manufacturer suggests is in four point contact including two points of contact with the bearing housing connected to the slide and two points with the rail. Besides being limited in its load carrying capacity by virtue of its relying on steel balls as bearing elements, this assembly requires an unique custom track or rail which adds manufacturing costs. Additionally, it is difficult to set such bearings up to have a predetermined amount of preload and accurate position with respect to the table and the rail to allow for free movement of the rollers within grooves in the rail.
Mounting of a plurality of separate dumbbell-roller bearing assemblies of the type shown in U.S. Pat. Nos. 3,003,878 to Stark or 3,343,262 to Kalmanek has also been suggested. Such a system advantageously can carry a heavy load with low friction. Because of cost, however, of multiple bearing assemblies themselves and of the mounting costs involved with individual mounting and alignment, such a system is not favored in many applications.
Accordingly, prior art linear bearing assemblies have substantial limitations and disadvantages which make them undesirable for machine tool applications.